Frequency shift keyer with automatic frequency control



Jan. 26, 1965 N. H. YOUNG, JR., ErAL FREQUENCY SHIFT KEYER WITH AUTOMATIC FREQUENCY CONTROL Filed March 17. 1961 IN VEN TORSI M @R Hm T mi N ,.snv mq MP wp United States Patent Ofce 3,167,712 Patented `Farr. 26, 1965 FREQUENCY SHIFT KEYER WlTH AUTMATKI FREQUENCY CGNTROL Norman H. Young, Jr., and Philip K. Spain', Nutley, N1., assignors to International Telephone and Telegraph Corporation, Nutley, NJ., a corporation of Maryland Filed Mar. 17, 1961, Ser. No. 96,478 18 Claims. (Ci. S25-163) This invention relates to communication systems and more particularly to an improved frequency shift keyer for `a frequency shift keyed communication system.

With an increased use of the frequency spectrum for communication purposes, the requirements for frequency precision have increased. Thus, it becomes important to eliminate as many components as possible that contribute to the frequency drift of the signal. Particularly in frequency shift keyed communication systems it is desirable that the center frequency, that is, the average or arithmetic mean of the two frequency signals transmitted, be exactly the assigned channel frequency. This requires both exact symmetry of deviation with respect to the mean frequency and an accurately controlled magnitude of deviation.

One arrangement for a frequency shift keyer is to employ a reactance modulator in combination with a variable frequency oscillator .to shift the frequency between two predetermined frequencies in accordance with a frequency control signal in the form of a keying signal or a coded intelligence signal varying between precisely fixed amplitude limits. This arrangement has the disadvantage that the frequency shift oscillator must be checked periodically to insure that it is accurate, that neither the magnitude of deviation nor the symmetry of deviation is inherently closely controlled, and that the amplitude of the keying signal must be accurately controlled.

Therefore, an object of this invention is to provide a frequency shift keyer that will provide an accurate control of both symmetry and magnitude of deviation from the mean frequency.

Another object of this invention is to provide a frequency shift keyer employing a reactance modulator and variable frequency oscillator combination that does not require an accurate control of the amplitude of the keying signal to provide an accurate control of both symmetry and magnitude of deviation from the mean frequency.

In accordance with the principles of this invention, a variable frequency signal has its frequency shifted betweentwo predetermined frequencies in accordance with a coded intelligence or keying signal and the frequency of the resultant variable frequency signal is locked to the predetermined frequencies by an arrangement responsive to the resultant variable frequency signal. This latter arrangement includes a mixer to heterodyne the resultant variable frequency signal with a rst reference signal and a comparator to compare the resultant beat signal output of the mixer with a second reference signal to produce a frequency control signal. The resultant frequency control signal is coupled under control of the coded intelligence signal to a frequency control means associated with the source of variable frequency signal to lock the frequency of the resultant variable frequency signal to the predetermined frequencies.

In accordance with a more specific aspect of this in-v vention, a bias voltage, also under the control of the coded intelligence signal, is provided to shift the frequency of the resultant variable frequency signal to atl locks the frequency of .the resultant variable frequency signal to the predetermined frequencies.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing, in which the single figure is a schematic diagram of a frequency shift keyer in accordance with the principles of this invention.

Referring to the ligure, the frequency shift keyer of this invention basically includes a source of variable frequency signal, such as, variable frequency oscillator 1, having the frequency of its output signal shifted between two predetermined frequencies in accordance with the coded intelligence or keying signal of source 2. Source Z may be any known device producing a signal having one of two signal levels or signal conditions, such as a telegraph key, facsimile equipment, teleprinter equipment, or the like. Means 3 cooperates in response to the out- V put signal of oscillator 1 to lock the frequency of the resultant variable frequency signal to the predetermined frequencies, one frequency representative of one signalling condition, such as a space in a telegraph like signal, and the other frequency representative of the other signalling condition, such as a mark in a telegraph like signal.

More specifically, variable frequency oscillator 1 includes `a resonant circuit comprising inductance 4, capacitors 5 and 6, and variable capacitor '7. Variable capacitor 7 may be used to adjust the frequency of the oscillator and, hence, establish a desired center frequency. Additional reactance is added to and subtracted from the resonant circuit of oscillator 1 by balanced modulator 8 including electron discharge devices 9 and 10. In accordance with this arrangement, by varying the voltages on grids 11 and 12 of devices 9 and 1t), respectively, the frequency of the oscillator 1 may be shifted a given amount below its center frequency and a similar amount above its center frequency to thereby provide the desired frequency shift signal. The resultant frequency of the output signal of oscillator 1 will be determined by the coded intelligence signal of source 2.

To facilitatethe description of the frequency shift keyer of this invention, let us assume that the proper combination of voltages exists on grids 11 and 12 of devices 9 and 1li, respectively, to cause the frequency of oscillator 1 to be below its center frequency fc, by an amount Af and that the signal 13 of source 2 is at its lower signalling condition, indicated for the purposes of explanation to be 0. With the illustrated schematic diagram of reactance modulator 3 the frequency of oscillator 1 will be decreased if the capacitive reactance current in device 9 is larger than the inductive reactance current in device 11i. This current relationship can be obtained by cou` pling ,a bias voltage to grid 11 which is positive with respect to a bias voltage applied to grid 12. In accordance with the figure, bias voltages to be coupled to grids 11 and 12 are obtained from a bias source 14 under control of signal 13 of source 2 through lthe medium of keyed polarity reversing switch 15. Switch 15 is illustrated for purposes of explanation to include a relay 16 havingfa solenoid 17 coupled to source 2 and four armatures 18, 19, 20 and 21. Source 14 includes a source of positive potential -l-v coupled to terminal 22 and a source of negative potential -v coupled to terminal 23 with potentiometers 2d and 25 coupled respectively to terminals ZZiand 23. The first assumption is satisfied when relay 1e is unenergized in the presence of the zero level of signal 13, that is, when armatures 18, 19, 2t), and 21 are positioned as illustrated. Thus, the positive bias voltage is coupled from potentiometer 24through armature 19 to resistor 26 and, hence, to grid 11 and the negative bias voltage is coupled from potentiometer 25 through armature i8 to resistor 27, and hence, `to grid l2. It may therefore be stated that the bias Voltage of source ld is a frequency control signal that at least adjusts the frequency of the output signal of oscillator l to approximately a value Af removed from the center frequency fc which with'the above assumption is a value equal to f Af.

c The frequency-shifted .output signal of oscillator 1 1s maintained at fC-Afin accordance with this invention by coupling at least a portion of thefrequency-shifted out- -put signal to mixer 2S where it is heterodyned with a `reference,frequency signal from oscillator 29. The siging a frequency Af. The comparison ofthese two sig-y ;nals produces a frequency control signal varying in accordance, with the phase rdiderence between the two sig-y nals. When the output signal of filter Sil' and the output signal of oscillator 32 are in phase, the output of cornparator 3l will be zero, indicating that the resultant fretquency-shifted signal output of oscillator l is lockedto` the predetermined frequency fc-Af. If there is a phase difference between the output signal of filter and the f output signal of oscillator 32, a correcting frequency sigi nal will be coupled through armatures 20 and 2l to the grids llZvand vl1, respectively, superimposed upon thebias f voltage from source 14 to correct the frequency` of the Y output signal of oscillator l to maintain the output signal locked to the predetermined frequency JCC-Af. By making oscillator 32 adjustable. the frequency deviation can be adjusted by .adjustment of oscillator 32.

When signal4 13 of source 2 assumes itshigher signalling-condition, indicated for purposes of explanation to be solenoid 17 is energized, causing armatures 13, 19,

, 2@ and 21 to assume their other contact position. Under this condition the polarity of the, bias voltage of source 14 and the sense of the error signal, if any, from phase n comparator 3l are simultaneously reversed .causingthe Cil l invention in connection with specific apparatus, it is to be clearly understood that this descriptionis made only byk way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanyingclaims.V

We claim:

l. A frequency shift keyer lcomprising a source of variable frequency signal, control means coupled to said source of variable frequency signal to shift thev frequency of said variable frequency signal between two predetermined frequencies in accordance with a coded intelligence signal, a source of reference signalhaving a given frequency, means coupled to said source of variable frequency signal to produce a resultant signal having va frequency related to 'said given frequency, means vresponsive' to said reference signal vandsaid resultant signal to produce a control signal `varying inV accordance with the `phase different between said referencesignal and vsaid,

resultant signal, andmeansresponsive to said intelligence signal to couple said control signal to' saidcontrol means to accurately -position the frequency` of said variable frequency signal at said predetermined frequencies.

2. A frequency shift keyer comprising a source of variable frequency signaha source of first reference signal, a` source of second reference signal having a given frequency, control means coupled to said source of variable frequency signal to shift the frequency of said variable frequencyv signal between two selected frequencies in accordance with a coded intelligence signal, each of said I selected'frequenciesl being spaced from Va predetermined means frequency by an amount vequal to said given frequency, means responsive to saidiirst reference signal and said variable frequency signal to produce a resultant signal having a frequency related to'said given frequency, means responsive to said second reference signal and said resultant signal to producel a control signal varying in accordance with thephase difference between said secoscillator to shift its frequency from one predetermined.

n frequency fc-Af, to a second predetermined frequency,

fc-knf. Should the bias voltage and the error voltage be such that the frequency of the output signal of oscillator l is slightly different than the predetermined frequency,

fc4-Af, that is, the beat signal outputof filter Sllsnot in phase with and, hence, has a slightly different frequency than the output'signal of voscillator 32, phase comparator 3l will produce a correcting frequency control signal for application tofreactance modulator 8 having the proper sense .and magnitude to lock the signal output of oscillator l tothe desired predetermined frequency Jc-l-Af. f The action of the frequency shift key of this invention is identical whether the frequency of the output signal of oscillator 1 goes from fc-Af to jc-i-Af or from fc4-Af to )-A].v It is obvious that with the corresting frequency control signal from comparator 31 thatr the mean frequency of the two predetermined frequencies is stable, that the magnitude of the deviation is constant, and that the amplitude of the keying signal, signal 13neednot be A controlled to change between two fixedy levels.

plitude of the keying signal only need .be suicient to f The amtrigger switch 15. I

Relay 16 is illustrated las one form of keyed polarity reversing switch 15, kbut it is tobe remembered that the switching actionmay be. executed electronically, partic- Lula'rly where there is a requirement of speed in excess of that which can be obtained with an electromechanical re-v lay.

While We have described above the principlesl of our ond reference signal and said resultant signal, and means responsive to said intelligence signal to couple said conftrol signal to said control means .to accurately Vposition Avthefrequency of the resultant lfrequency-sllifted signal `output ofk saidV source of variable frequency signal at said selected frequencies. n

'3. A frequency shift keyer comprisingv a source of variable frequency signal, a source of iirst'reference signal, a source of second reference signalhaving a given frequency, control means coupled to said source 'of variable frequency signal to shift'the frequency ofs'aid variable frequency. signal'between two selected frequencies in f accordance with a coded intelligence signal, each of said selected frequencies being spaced from a predetermined means frequency by an amount equal to saidgiven frequency, amixer coupled to smd source of variable frequency signal and said source .of first reference signal to produce a beat signal having a frequency related to said given` frequency, means responsive to said second reference signal and said beat signal Ito produce a control sig- 4nal varyingin accordance with the phase difference between said second reference signal and said beat signal, and means responsive to said intelligence signal to couple said control signaltto said control means toaccurately position the frequency of the resultant frequencyshifted signal output of said source of variable frequency signalat said selected'frequencies.

A vfrequency shiftkeyer comprising a source of varlable frequency signal, a source of rst reference signal, a source of'second reference signal having a given frequency, a reactance modulator coupled to said source ,y of yaria'ole frequency signal'to shift the frequency of said variable frequency signal between two selected frequencies in accordance with a'coded intelligence signal, each of said yselected frequenciesrbeing spaced from a predetermined mean frequency by an amount' equal V.to 'said given frequency, a mixer coupled ,togsaid source of first reference ignal ,and-said sourceof variable frequency signal to produce a beat signal having a frequency related to said given frequency, means responsive to said second reference signal and said beat signal to produce a control signal varying in accordance with the phase difference between said second reference signal and said beat signal, and means responsive to said intelligence signal to couple said control signal to said reactance modulator to accurately position the frequency of the resultant frequencyshifted signal output of said source of variable frequency signal at said selected frequencies.

5. A frequency shift keyer comprising a source of variable frequency signal, Aa source of iirst reference signal, a source of second reference signal having a given frequency, a balanced reactance modulator coupled to said source of variable frequency signal to shift the frequency of said variable frequency signal between two selected frequencies in accordance with a coded intelligence signal, each of said selected frequencies being spaced from a predetermined mean frequency by an amount equal to said given frequency, a mixer coupled to said source of first reference signal and said source of variable frequency signal to produce a beat signal having a frequency equal to said given frequency, a phase comparator coupled to said source of second reference signal and said mixer to produce a control signal varying in accordance with the phase difference between said second reference signal and said beat signal, and means responsive to said intelligence signal to couple said control signal to said reactance modulator to accurately control the frequency of the resultant lfrequency-shifted signal output of said source of variable frequency signal at said selected frequencies.

6. A frequency shift keyer comprising a source of variable frequency signal having a predetermined center frequency, a source of first reference signal having a frequency equal to said center frequency, a source of second reference signal having a given frequency, a balanced reactance modulator coupled to said source of variable frequency signal to shift the frequency of said variable frequency signal between two selected frequencies in accordance with a coded intelligence signal, each of said selected frequencies being spaced symmetrically from said center frequency by an amount equal to said given frequency, a mixer coupled to said source of first reference signal and said source of variable frequency signal to produce a beat signal having a frequency related to said given frequency, a phase comparator coupled to said source of second reference signal and said mixer to produce a control signal varying in accordance with the phase difference between said second reference signal and said beat signal, and switching means responsive to said intelligence signal to couple said control signal to said reactance modulator to accurately control the frequency of the resultant'frequency-shifted signal output of said source of variable frequency signal at said selected frequencies.

7. A frequency shift keyer comprising a source of variable frequency signal, a frequency control means coupled to said source, means coupled to said source including at least means responsive to the signal of said source to provide a frequency control signal, a switching means coupled to said responsive means and said control means, and a source of coded intelligence signal coupled to said switching means to control the coupling of said control signal to said control means to shift the frequency of said variable frequency signal between two predetermined frequencies in accordance with said intelligence signal and to maintain the frequency of said variable frequency signal at said predetermined frequencies.

8. A frequency shift keyer comprising a source of variable frequency signal, a reactance modulator coupled to Y said source of variable frequency signal, means coupled to said source of variable frequency signal including at least means responsive to said variable frequency signal to provide a frequency control signal, a polarity reversing l switching means coupled to said responsive means and said reactance modulator, and a source of coded intelligence signal coupled to said switching means to control the coupling of said control signal to said Vreactance modulator to shift the frequency of said variable frequency signal between two predetermined frequencies in accordance with said intelligence signal and to maintain the frequency of said variable frequency signal at said predetermined frequencies.

9. A frequency shift keyer comprising a source of variable frequency signal, a reactance modulator coupled to said source of variable frequency signal, a source of reference signal having a given frequency, means coupled to said source of variable frequency signal to produce a resultant signal having a frequency related to said given frequency, means responsive to said reference signal and said resultant signal to provide a frequency control signal varying in accordance with the phase difference between said reference signal and said resultant signal, a polarity reversing switching means coupled to said responsive means and said reactance modulator, and a source of coded intelligence signal coupled to said switching means to control the coupling of said control signal to said reactance modulator to shift the frequency of said variable frequency signal in accordance with said intelligence signal between two selected frequencies each spaced from a predetermined mean frequency by an amount equal to said given frequency and to maintain the frequency of the resultant frequency-shifted signal output of said source of variable frequency signal at said selected frequencies.

l0. A frequency shift keyer comprising a source of variable frequency signal, a reactance modulator coupled to said source of variable frequency signal, a source of first reference signal, a source of second reference signal having a given frequency, means responsive to said -iirst reference signal and said variable frequency signal to produce a resultant signal having a frequency related to said given frequency, means responsive to said second reference signal and said resultant signal to provide a `frequency control signal varying in accordance with the phase difference between said second reference signal and said resultant signal, a polarity reversing switching means coupled to said responsive means and said reactance modulator, and a source of coded intelligence signal coupled to said switching means to control the coupling of said control signal to said reactance modulator to shift the frequency of said variable frequency signal in accordance with said intelligence signal between two selected frequencies each spaced from a predetermined mean frequency by an amount equal to said given frequency and to maintain the frequency of the resultant frequency-shifted signal output of said source of variable frequency signal at said selected frequencies.

1l. A frequency shift keyer comprising a source of variable frequency signal, a reactance modulator coupled to said source of variable frequency signal, a source of first reference signal, a source of second reference signal having a given frequency, a mixer coupled to said source of Evariable frequency signal and said source of first reference signal to produce a beat signal having a frequency related to said given frequency, means responsive to said second reference signal and said beat signal to provide a frequency control signal varying in accordance with the phase difference between said second reference signal and said beat signal, a polarity reversing switching means coupled to said responsive means and said reactance modulator, and a source of coded intelligence signal coupled to said switching means to control the coupling of said control signal to said reactance modulator to shift the frequency of said variable frequency signal in accordance with said intelligencev signal between two selected frequencies each spaced from a predetermined mean frequency by an amount equal to said given frequency and to maintain the frequency of the resultant frequencyshiftedsignal output of said source of variable frequency signal at said selectedfrequencies.

f to said source of variable frequency signal and said source of first reference signal to produce a beat signal having .a frequency related to said given frequency, a phase comparator responsive to said second` reference signal and said beat ksignal to provide a frequency control signal. *varying in accordance with the phase difference between said reference signal andpsaid beat'signal, a polarity reversing switching means coupled to said phase comparator and said reactance modulator, and a source of coded intelligence signal coupled to said switching means to control the coupling of said control signal to said reactance modulator to shift the frequency of said variable frequency signal in accordance with said intelligence signal between two selected yfrequencies eachV spaced from a pre- Y determined mean frequency by an amount equal to said given frequency yand to ymaintain the frequency of the resultant frequency-shifted` signal output of said source of variable frequency signal at said selected frequencies.

13. A frequency shift keyer comprising a source of variable frequency signal, `a frequency control means coupled to said source of variable frequency signal, a bias source to provide a first frequency control signal, means responsive to said variable frequency signal to provide` a second` frequency control signal, a switching means coupling said responsive means and said .bias source to said control means, and a source of coded intelligence signalcoupled to said switching means to` control-in accordance with said intelligence signal the coupling of said iirst control signal to said control means to shift the frequency of said variable frequency signal between two predetermined requencies and said second control signal to said control means to maintain the frequencyof said variable frequency signal at said predetermined frequencies.

14. A frequency shift keyer comprising a source of variable frequency signal, a reactancemodulator coupled to said source of variable frequency signal, a bias source to provide a first frequency control signal, means responsive n to said variable frequencyy signal to'provide a second frequency control signal, a polarity reversing switching means coupling said responsive means and said bias source to said reactance modulator, and asource of coded intelligence signal coupled to saidswitching means to control in accordance with said intelligence signal the coupling of saidtiirst control signal to said reactance modulator to shift thefrequency of said `variable frequency. signal between two predetermined 'frequencies and said second control signal to said reactance modulator to maintain` the frequency of said variable frequency` signal at said predetermined frequencies.

15. A frequency shift lceyer comprising asource of variable frequency` signal, a' reactance modulator coupled to said resultant signal to provide a second frequency control. i

signal varying in accordance with the phase difference between said reference signal and said resultant signal, a polarity reversing switching means coupling said responsive means and said bias source to said reactance modu-` lator, and a source of coded intelligence signal coupled-` to said switching means to control in accordance with said intelligence signalthe coupling. of-Vsaid first control signal to said reactance modulator to shift the frequency of said` c variable frequency signal between two selected frequencies each spaced from a predetermined mean frequency-by an amount equal to said given frequency and said second consaid sourcek of variable frequency signal, Va bias source to trol signal to-said reactance modulator to. maintain the frequency of the resultant.frequency-shifted signal output of said source of variable frequency signal Vat said selected frequencies.

16. A frequency shift keyer comprising a source of variable frequency signal, a reactauce modulator coupled to said `source of variable frequency signal, a bias source to provide a lirst frequency control signal,` a source of first :reference signal, a sourceof second reference signal having a given frequency, means responsive to said first reference signal and said variable frequency signal to produce a resultant signal haviug'a frequency related to said given frequency, means responsive to said second reference signal andsaid resultant signal to provide a second frequency control signal varying in accordance with the phase difference between said reference signal and said resultant signal, a polarity reversing switching means coupling said responsive means andv said bias source to said reactance modulator, and a source of coded intelligence signal coupled to said switching meansV to control in accordance vwith said intelligencesignal the coupling of said'first control signal to said reractance` modulatorto shift the frequency of said variable frequency signal between-two selected frequencies; each spaced from a predetermined mean frequency by` an .amount equal to said given frequency and'saidk second control signal to said reactance modulator to'maintain the frequency of the resultant frequency-shifted signal output of said source of rfrequency signal at said selected frequencies.

17. A frequency shift. keyer comprising a source of variable frequency signal, areactance Vmodulator coupled to said sourceaof `variable frequency signal, -a bias source to provide a first frequency control signal, a source of first reference signal, a source of second reference signal having a givenfrequency, a mixer coupled to said source of variable frequency signal andsaid source of first reference signal to` produce a beat signal having a frequency v relatedk tosaid givenv frequency, means responsive to said determined mean frequency by anramountequal to said givenffrequency and said second `controlsignal to said reactance modulator to maintain the frequency of the resultant frequency-shifted :signal output' of said source of variable frequency signal at said selected frequencies.

185A frequency rshift. keyer comprising aV source` of variable frequency signal,.a balanced reactance modulator coupled to saidysourceofvariable frequency'signal, a bias source to provide a first frequency control signal, a source of first reference signal, a source of second reference signal havinga given frequency, a mixer rcoupled to said source of variablefrequency signaltand said source of saidfirst reference signal to produce a beatl signal having a frequency related tousaid given frequency, a phase comparator responsive to; saidsecondhreference signaland said beat signal toprovideva second frequency control signal varying in accordance with the phase dif- `ferencebetween said referencesignal-and said beat signal,

a polarity reversing switching means coupling said phase comparator andl said bias source `to Asaid reactance modulator, and' a source of coded intelligence signal coupled to said-switching means to control ini accordance with f l@ frequencies each spaced from a predetermined mean fre- References Qied in the file of this patent quency by an amount evzpunl.v to said given frequency and UNTED STATES PATENTS said second control signal to said rcacance modulator to e maintain the frequency of ythe resultant frequency-shifted y 2,492,795 Goldstme Dec 27, 1949 signa output of said source of Variable frequency signal 5 2,689,881 Leypod e- Sept' 21, 1954 

7. A FREQUENCY SHIFT KEYER COMPRISING A SOURCE OF VARIABLE FREQUENCY SIGNAL, A FREQUENCY CONTROL MEANS COUPLED TO SAID SOURCE, MEANS COUPLED TO SAID SOURCE INCLUDING AT LEAST MEANS RESPONSIVE TO THE SIGNAL OF SAID SOURCE TO PROVIDE A FREQUENCY CONTROL SIGNAL, A SWITCHING MEANS COUPLED TO SAID RESPONSIVE MEANS AND SAID CONTROL MEANS, AND A SOURCE OF CODED INTELLIGENCE SIGNAL COUPLED TO SAID SWITCHING MEANS TO CONTROL THE COUPLING OF SAID CONTROL SIGNAL TO SAID CONTROL MEANS TO SHIFT THE FREQUENCY OF SAID VARIABLE FREQUENCY SIGNAL BETWEEN TWO PREDETERMINED FREQUENCIES IN ACCORDANCE WITH SAID INTELLIGENCE SIGNAL AND TO MAINTAIN THE FREQUENCY OF SAID VARIABLE FREQUENCY SIGNAL AT SAID PREDETERMINED FREQUENCIES. 